The present invention relates generally to ion implantation systems, and more specifically to methods and apparatus for improved ion bunching in an ion implantation system.
In the manufacture of semiconductor devices, ion implantation is used to dope semiconductors with impurities. A high energy (HE) ion implanter is described in U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, which is hereby incorporated by reference as if fully set forth herein. Such HE ion implanters are used for deep implants into a substrate in creating, for example, retrograde wells. Implant energies of 1.5 MeV (million electron volts), are typical for such deep implants. Although lower energy may be used, such implanters typically perform implants at energies between at least 300 keV and 700 keV. Some HE ion implanters are capable of providing ion beams at energy levels up to 5 MeV.
Referring to FIG. 1, a typical high energy ion implanter 10 is illustrated, having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22. The ion source 20 produces an ion beam 24 that is provided to the beamline assembly 14. The ion beam 24 is then directed toward a target wafer 30 in the end station 16. The ion beam 24 is conditioned by the beamline assembly 14 that comprises a mass analysis magnet 26 and a radio frequency (RF) linear accelerator (linac) 28. The linac 28 includes a series of accelerating modules 28a-28n, each of which further accelerates ions beyond the energies they achieve from prior modules. The accelerating modules maybe individually energized by a high RF voltage that is typically generated by a resonance method to keep the required average power reasonable. The mass analysis magnet 26 passes only ions of an appropriate charge-to-mass ratio to the linac 28.
The linear accelerating modules 28a-28n in the high energy ion implanter 10 individually include an RF amplifier, a resonator, and an energizable electrode. The resonators, for example, as described in U.S. Pat. No. 4,667,111 operate at a frequency in the range of about 3-30 Mhz, with a voltage of about 0 to 150 kV, in order to accelerate ions of the beam 24 to energies over one million electron volts per charge state. As the ion beam 24 travels through the various accelerating modules is or stages 28, some of the ions therein are properly accelerated, whereas others are not. Thus, conventional linear accelerators 28 achieve an ion acceleration efficiency that is less than 100%. In particular, conventional ion accelerators may deliver less than 20% of the ions from the mass analysis magnet 26 to the target wafer 30 in the end station 16. In particular, each ion accelerating stage 28a-28n is tuned or adjusted in order to provide appropriate acceleration to ions provided thereto which fall within a tolerance or acceptance range. Maintaining ions in an ion beam is facilitated where the ions are bunched or packetized prior to acceleration, whereby a larger percentage of ions are accelerated by each accelerating module or stage 28. In conventional linear accelerators 28, the first or the first and second accelerating modules (e.g., modules 28a, 28b) may operate as a combination buncher and accelerator. However, this bunching provides limited ion transfer efficiency. Thus, typical linear accelerators 28 may achieve less than 20% transmission of ions. In ion implantation devices, it is desirable to impart ions onto a workpiece, such as a semiconductor product, in a controlled fashion. In conventional systems where approximately 80% of ions generated by an ion source therein may be lost (e.g., not provided to the workpiece), more time is needed to perform the desired implantation. Thus, there is a need for improved methods and apparatus for bunching ions in an ion implantation linear accelerator, in order to increase the percentage of generated ions that are imparted onto a workpiece.
The present invention is directed to a linear accelerator having an ion buncher associated therewith that achieves improved ion transport in an ion implantation system. The invention provides a dedicated buncher stage for ensuring that a greater percentage of the generated ions are provided in the region of acceleration for the accelerating stage of the ion implanter, than was heretofore possible. The buncher stage may be positioned upstream of a linear accelerator stage along a beam path in an ion implantation system in order to provide bunches or packets of ions to the accelerating stage of the accelerator. In particular, the invention provides for beam transmission of up to 60%, for example, of the available ions through the linear accelerator of an ion implantation system. Thus, the invention provides significant advantages over conventional ion implantation devices and methodologies in which in some cases less than 20% of available ions were properly accelerated.
One aspect of the invention provides an asymmetrical double gap buncher providing further advantages and efficiencies associated with ion transfer in an ion implantation system. First and second gaps are provided before and after a buncher modulating electrode, respectively, wherein the gaps differ in size, for example, wherein the second gap is larger than the first gap.
The modulating field in the buncher accelerates certain ions with respect to a reference ion, and decelerates others with respect thereto. In the drift region, the accelerated ions catch up to the reference ion and the decelerated ions slow down to allow the reference ion (e.g., as well as the accelerated ions) to catch up, thereby providing a net bunching effect. The asymmetry of the gaps facilitates the provision of a higher percentage of available ions to subsequent linear accelerator stages, thereby significantly improving the ion transfer efficiency of ion implantation systems. The asymmetrical double gap buncher may be included as a buncher stage within a linear accelerator system for an ion implanter in which the modulating field strength created by the buncher electrode may be significantly lower than that of the accelerating alternating electric field of the linear accelerator. In this manner, the buncher electrode operates to modulate the DC ion beam (e.g., obtained from an upstream mass analysis magnet), and the drift region allows bunching to occur as a result of the modulation.
In accordance with a further aspect of the invention, there is provided a slit double gap buncher stage and a modulating electrode therefor, which provide further advantages associated with ion implantation. The modulating electrode comprises an elongated slit aperture in an electrode base extending longitudinally through the base along the ion beam path. The slit may comprise an aspect ratio greater than one, for example, in which the slit height is greater than the slit width. The slit aperture allows reduced gap lengths in the buncher compared to circular electrode apertures, resulting in efficient modulation over a wider range of ion velocities. In addition, the slit double gap buncher may be located between matching quadrupole focusing devices and an entrance aperture in a linear accelerator. In this case, the matching quadrupoles may serve to form the bunched ion beam into a circular profile for injection into the first accelerating stage, as well as to provide a buncher drift region. This provides for reduction in length for an ion implantation device.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.